Indirectly heated cathode



5, 1933- K. M. VAN GESSEL INDIRECTLY HEATED CATHODE Filed Feb. 24, 1950 INVENTOR K.M- VAN GESSEL ATTORNEY Patented Dec. 1933 UNITED STATES PATENT OFFICE lands, assignor to Radio Corporation of America, a corporation of Delaware Application February 24, 1930, Serial No. 430,590, and in the Netherlands April 13, 1929 3 Claims.

The invention relates to electric discharge tubes comprising an indirectly heated cathode. During the operation of discharge tubes there is often the disadvantage that owing to variation in 5 length of the cathode due to intense heat, the

cathode is liable to undue deformations, for example warping. This may give rise to variations in the characteristic values of the discharge tube and to short-circuits between the cathode and the other electrodes enclosed within the tube.

The invention has for its object to obviate said difiiculties. According to the invention one end of the indirectly heated cathode is supported in such a manner that it can freely move longitudinally through a guide which preferably comprises a support having an aperture in which the end of thecathode fits loosely.

Theinvention will be more clearly understood by reference to the accompanying drawing, illusgo trating by way of example, an electric discharge tube according to the invention.

The discharge tube shown in the figure comprise a bulb 1 which has sealed to it a stem 2 comprising a pinch 3 and having mounted on it a plurality of electrodes. The tube comprises an indirectly heated cathode 4 constituted by a cylindrical sleeve or tube of small diameter, say of nickel, the outer surface of which is coated with a substance of high electron emission, for example, one or more of the alkali-earth metal oxides. The tube 4, which is closed at one end, contains a heating element 5 constituted by a U-shaped wire, for example, of tungsten. The two legs of this heating element are insulated from one another and from the cylinder 4 by an insulation 6 which fills the tube 4 and fastens it to the heating element 5 and consists, for example, of aluminum oxide. The ends of the heating element 5 are secured to lead wires 7 and 8 mounted on the pinch 3 and constituting a support for one end of the cathode. The closed end of the tube 4 is supported by another support 9 which is also a lead and constitutes a guide for the end of the cathode by reason of an aperture. 10 in the support 9 in which the closed end of the tube fits loosely so as to move freely as the cathode expands and contracts. A good electrical connection between the support 9 and the tube 4 is ensured by a flexible connection 11.

The cathode 4 is surrounded by two cylindrical anodes 12 and 13 mounted on the pinch by means ofsupporting wires 14 and 15. The various conductors are taken through the pinch 3 and connected to contact pins 16 of a base 17.

Owing to the fact that the closed end 01' the tube 4 can slide freely in the aperture 10, of the guide, the cathode will not warp if its length changes in the presence of intense heat but'the end of the cathode will be slightly displaced in the aperture 10. By reason of the cathode having a sturdy construction, it need not be kept taut with the aid of a resilient member as usually employed in the case of a directly heated cathode so that in the discharge tube according to the invention such resilient members are dispensed with and thus thediificulties incidental thereto are obviated.

Although the discharge tube illustrated is an exhausted one adapted for full-Wave rectification of alternating currents, it will, of course, be evident that the invention may be applied to other kinds of discharge tubes such as gas-filled tubes or tubescomprising a plurality of electrodes.

I claim: H

1. An integral indirectly heated cathode comprising a cylindrical sleeve having inside it a filamentary heating element with its terminals projecting side by side from one end of said sleeve, a pair of rigid leads fixed to the projecting terminals of said heating element and con- 0 stituting a support for one end of said cathode,

a third rigid lead constituting a cooperating support loosely engaging the other end of said cathode, and a flexible electrical connector between said third lead and said other end of said 35 cathode.

2. An integral indirectly heated cathode comprising a cylindrical sleeve having an internal heater element fixed to said sleeve, a pair of heater supporting wires at one end of said cathode, and a cathode lead-in wire comprising a guide in which the other end of said cathode is free to move.

3. An integral indirectlyheated cathode comprising a cylindrical sleeve closed at one end, an internal heating element fixed to said sleeve and extending from the open end thereof, a pair of heater supporting wires connected to said heating element at the open end of said sleeve, and a cathode lead-in wire provided on its free end with an aperture in which the closed end of said sleeve fits loosely enough to slide as said cathode expands and contracts.

KAREL MARINUS VAN GESSEL. 

